1. Field of the Invention:
The present invention relates to a reproducing apparatus of an optical recording medium and an optical pickup device, and more particularly to a reproducing apparatus of an optical recording medium which is capable of reproducing an information signal from at least two types of optical recording mediums using an optical pickup device.
2. Background of the Invention
In recent years, the recording density of an optical disk, which is capable of serving as a storage unit tor a computer and a package media for music and image information, has been increased. In order to increased the recording density, a variety of contrivances have been considered in which the NA (Numerical device is enlarged and in which the diameter of the beam spot is reduced by shortening the wavelength of the light beam.
In general, an optical disk using a wavelength of 635 nm or 650 nm, having a NA of 0.45 or greater and capable of recording data at a high density includes a substrate having a thickness of, for example, 0.6 mm which is smaller than the thickness of 1.2 mm of the substrate of the conventional optical disk in order to enlarge an allowable error (hereinafter called "tolerance") against deterioration of the information signal occurring due to skew or the like.
It is leads to a fact that plural types of optical disks having different specifications, such as formats and thicknesses of the substrates, exist on the market. However, users have a requirement for reproducing the plural types of the available optical disks by one optical-disk reproducing apparatus. A compatible optical disk reproducing apparatus has been considered to be employed which is capable of reproducing optical disks having different optical specifications, such as thicknesses of the substrates.
When the conventional optical pickup device reads and reproduces an information signal from an optical disk composed of a substrate having a thickness of 1.2 mm by an objective lens adapted to be capable of forming a beam spot having all optimum shape on an optical disk composed of a substrate having a thickness of 0.6 mm and having data recorded at a high density, spherical aberration that is generated and attributable to the difference in the thickness of the substrate raises a problem that a satisfactory information signal cannot be obtained.
Accordingly, a portion of the conventional optical pickup devices has a structure having a plurality of objective lenses consisting of a first objective lens adapted to the optical disk for use in a high recording density operation arid composed of the substrate having the thickness of 0.6 mm, and a second objective lens composed of the substrate having the thickness of 1.2 mm. Thus, either of the two objective lenses is switched to be adaptable to the objective lens from which information signals are attempted to be read and reproduced so that the information signals are read and reproduced. However, the above-mentioned optical pickup device has a problem in that the control and the structure for switching the objective lenses become too complicated and the manufacturing cost cannot be reduced.
Another portion of the conventional optical pickup devices uses a hologram to employ an objective lens having two focal distances, using either beam spot to be adaptable to the thickness of the substrate of the optical disk. However, the optical pickup device encounters a problem that the cost for manufacturing the objective lens having two focal distances cannot be reduced and reliability is unsatisfactory.